Resistivity is a key factor used in the investigation of a concrete structure, whether it is for QA during construction, to determine the rate of corrosion or determine the overall quality of concrete in-situ. It is completely non-destructive and measured using the Wenner Probe methodology, current is passed between the outside probes and the potential measured between the inner probes. The spacing (a), voltage (V) and current (I) are the used in the equation:
Resistivity ρ= 2πaV/l [kΩcm]
Resistivity as a QA Tool
The Rapid Chloride Permeability (RCP) test was originally introduced in the 1990’s as a means of assessing reinforcement corrosion protection provided by concrete. Originally, it was thought to combine a measure of chloride diffusion and resistance to current flow and hence, give a result that could be used to assess corrosion protection afforded through the initiation and propagation phases. In Australia chloride diffusion tests were already being used to assess concrete performance but the tests took a long time, and had by then been proven to only give information on the initiation phase. Also there were no simple criteria.
Later there were criticisms of the RCP on the basis that it was primarily a measure of resistivity and that some additives could give results indicating poor corrosion resistance whereas the additive increased corrosion resistance, e.g. calcium nitrate. However, the RCP has remained a popular quick QA tool for assessing concrete corrosion resistance. As a QA tool the RCP test criteria can be set for the specific concrete mix at the time of the mix trials.
A study initiated in 2002 by the Florida Department of transport has determined a strong correlation between Rapid Chloride Penetration [RCP] measurements and saturated resistivity measurements taken on the same samples (see image left). This discovery means testing can now be done in 10 minutes instead of 10 days using the same cylinders or cores used to determine compressive strength. To solidify this ASSHTO have drafted the Surface Resistivity Test specification to standardise the test.
Replacement or augmentation of the RCP test with Surface Resistivity tests of saturated samples has the added benefit of lower costs and greater volume of tests. It is easy to visualise the concrete lab removing the test cylinder from the water bath, placing the Resistivity meter on the cylinder and then placing it in the compression testing machine for crushing.
Resistivity as a Corrosion Investigative Tool
Resistivity is also one of the key controlling factors once corrosion begins (i.e. the propagation phase). Corrosion requires a flow of ions between anodic and cathodic sites on the reinforcement of concrete; the resistivity controls the rate of this flow of ions and therefore directly controls the rate of corrosion. What this means is that by measuring resistivity it is possible to determine if a structure that is corroding and will deteriorate quickly or if the corrosion rate is slow and can be addressed in a more conservative manner. This can be a very useful test in combination with Half-Cell potential measurement to locate hot spots for corrosion likelihood.
It has been recognised that resistivity is also related to chloride ingress in the initiation phase through the Nernst-Einstein equation and also directly related to the propagation phase as it controls the corrosion rate in many situations.
Use in the Field
The presence of rebar disturbs electrical resistivity measurements as they conduct current much better than the surrounding concrete. This is particularly the case when the cover depth is less than 30mm. As far as possible, reinforcement bars should not be directly beneath the probe and should not run parallel to the probe. The recommended measurement orientation is determined by the spacing of the rebar compared with the probe spacing.
The optimum orientation is to measure diagonally to the bar as shown. This is possible if the probe span is less than the rebar grid spacing. If the rebar spacing is so close that it cannot be avoided, the influence of the steel can be minimised by measuring perpendicular to the rebar as shown. RILEM TC154-EMC: ELECTROCHEMICAL TECHNIQUESFOR MEASURING METALLIC CORROSION recommends making five readings from the same location moving the probe a few mm between each measurement and taking a median from the five values.
Introducing the All New Resipod for Resistivity Measurements
It is a one piece device which is rugged and water proof. The manufacturer has recognised a significant limitation of resistivity measurements was calibration and probe contact. The ResiPod has been designed to overcome both issues, special probe design and current compensation makes it a market leading device. It has full data logging capabilities with USB connection and reporting software included.